An immersion nozzle, a long nozzle, a tundish nozzle, a semi-immersion nozzle, etc. are known as nozzles for continuously casting steel.
An “immersion nozzle” will be described as an example of the nozzle for continuously casting steel. The purpose of use of the immersion nozzle is to seal a tundish and a mold from each other to thereby prevent re-oxidation of molten steel and to control a flow of molten steel out of a discharge hole of the immersion nozzle and uniformly supply molten steel into the mold to attain operating stability and improvement in cast piece quality.
As a method for controlling the flow rate of molten steel for supplying the molten steel into the mold through the immersion nozzle, there is known a stopper method or a slide plate method. Particularly, in the slide plate method, a set of two or three hole-including plates are used so that one of the hole-including plates is slid to adjust the flow rate on the basis of the aperture of the hole. Accordingly, if the aperture is small, a drift is apt to occur in the immersion nozzle. If such a drift occurs in the immersion nozzle, the flow rate out of each discharge hole becomes so ununiform that a drift occurs in the mold to deteriorate cast piece quality.
Prevention of the drift in the immersion nozzle is important in order to improve cast piece quality. As a technique for preventing the drift in the immersion nozzle, there is known a method of improving the shape of an inner hole portion of the nozzle. For example, “provision of ring-like protrusions” has been proposed as described in an “immersion nozzle (Patent Document 1) having a molten steel flow hole provided with a plurality of step portions”, an “immersion nozzle (Patent Document 2) having a molten metal introduction portion provided with a throttle portion to use a region of from the throttle portion to a discharge hole as a flow rate relaxing portion”, and a “continuous casting immersion nozzle (Patent Document 3) having four or more wavy folds each shaped like a circular arc and provided continuously in the flowing direction of molten metal in an inner surface of a nozzle hole so that the distance between adjacent peaks of the folds is from 4 to 25 cm and the depth between a peak and a corresponding trough is from 0.3 to 2 cm”. “Provision of helical protrusions” has been also proposed as described in a “casting nozzle (Patent Document 4) having an inner wall provided with spiral grooves or protrusions”, an “immersion nozzle (Patent Document 5) having an inner wall preferably provided with double-helical or triple-helical protrusions”, and so on. There have been further proposed a “nozzle (Patent Document 6) having semi-spherical concave-convex portions formed in a surface of a molten metal flow passage”, a “casting nozzle (Patent Document 7) having convex or concave portions in an inner surface of a nozzle hole so that the convex or concave portions are continuous in a direction perpendicular to the flowing detection of molten steel”, and an “immersion pipe (Patent Document 8) having a throttle ring disposed in a free transverse section of the immersion pipe to narrow the free transverse section of the immersion pipe and form a longitudinal section of the throttle ring to generate a laminar flow of molten metal in an outflow port, the throttle ring being disposed in the immersion pipe”.
On the other hand, when Al killed steel or the like is cast, a mainly alumina-containing non-metal inclusion (hereinafter referred to as “alumina” simply in this description) is generally attached and deposited on a molten steel flow hole portion surface (inner pipe surface) of the immersion nozzle. If the amount of alumina deposited on the inner pipe surface of the immersion nozzle becomes large, the operation becomes unstable because the increase in the amount of alumina causes narrowing of the nozzle inner hole portion, reduction in casting speed, drifting of a discharge flow, blocking of the nozzle inner hole, etc. Moreover, if part of the deposited alumina is dropped out by a flow of molten steel, penetrated into the mold and caught in a solidification shell, cast piece quality is lowered because of a large-size inclusion defect. As described above, “deposition of alumina” on the inner pipe surface of the immersion nozzle exerts a bad influence on both operation and cast piece quality as well as reduction in the lifetime of the nozzle. This phenomenon also occurs in other nozzles such as a long nozzle, a tundish nozzle, etc.
As general means for preventing alumina from being deposited in the casting nozzle, there is known a method of spraying inert gas. Generally, this method is a method of spraying inert gas from an insert nozzle or upper plate of a slide gate or from a stopper fitting portion of an insertion type immersion nozzle. When the cleanliness factor of molten steel is low, a method of spraying inert gas directly from the immersion nozzle is also carried out.
A material (alumina-deposition-free material) applied to the nozzle has been proposed in order to prevent alumina from being deposed on the casting nozzle. For example, provision of a boron nitride (BN)-containing material (Patent Document 9), a BN—C refractory material (the aforementioned Patent Document 1), or the like, in the inner hole portion of the immersion nozzle has been proposed. Provision of an Al2O3—SiO2—C material, a CaO—ZrO2—C material, a carbonless refractory material or the like has been further proposed.
A large number of proposals have been further made from the aspect of the shape of the inner hole portion of the casting nozzle. For example, besides the aforementioned Patent Documents 1 to 8, there have been proposed a “molten metal injection nozzle (Patent Document 10) having a plurality of grooves formed along the lengthwise direction of its inner wall in a region of the inner wall including a portion of collision with molten metal”, a “molten metal induction pipe (Patent Document 11) having an inner wall provided with at least one helical step and having a portion in which the sectional area of a molten metal flow path is reduced gradually in a region ranging from the inlet side to the outlet side”, a “continuous casting immersion nozzle (Patent Document 12) having a slit-like discharge hole in a bottom portion of the continuous casting immersion nozzle, and orifices in the inside of the nozzle, having a structure in which the shape of a planar section surrounded by each orifice is elliptical or rectangular or such a shape that each rectangular short side replaced by a circular arc to narrow a flow of molten metal flowing in the immersion nozzle, and formed so that the direction of each long side of the planar section surrounded by the orifice is perpendicular to the direction of each long side of a planar section of the slit-like discharge hole in the bottom portion”, an “immersion nozzle (Patent Document 13 or 14) having a twisted tape-like swirl vane for generating a swirl flow of molten steel in the nozzle and shaped so that the inner diameter of the nozzle is narrowed by a lower portion of the swirl vane”, and so on.
[Patent Document 1]: Japanese Utility Model Publication No. 23091/1995 (Claims 1 and 5)
[Patent Document 2]: Japanese Patent No. 3,050,101 (Claim 1)
[Patent Document 3]: Japanese Patent Laid-Open No. 269913/1994 (Claim 1)
[Patent Document 4]: Japanese Patent Laid-Open No. 130745/1982 (Scope of Claim for a Patent)
[Patent Document 5]: Japanese Patent Laid-Open No. 47896/1999 (Claims 1 and 2)
[Patent Document 6]: Japanese Patent Laid-Open No. 89566/1987 (Claim 1 in Scope of Claim for a Patent)
[Patent Document 7]: Japanese Utility Model Publication No. 72361/1986 (FIGS. 2 to 4)
[Patent Document 8]: Japanese Patent Laid-Open No. 207568/1987 (Claim 1 in Scope of Claim for a Patent)
[Patent Document 9]: Japanese Utility Model Publication No. 22913/1984 (Scope of Claim for a Utility Model Registration)
[Patent Document 10]: Japanese Patent Laid-Open No. 40670/1988 (Claim 1 in Scope of Claim for a Patent)
[Patent Document 11]: Japanese Patent Laid-Open No. 41747/1990 (Scope of Claim for a Patent)
[Patent Document 12]: Japanese Patent Laid-Open No. 285852/1997 (Claim 2)
[Patent Document 13]: Japanese Patent Laid-Open No. 2000-237852 (Claim 1)
[Patent Document 14]: Japanese Patent Laid-Open No. 2000-237854 (FIGS. 1 to 3)
In the aforementioned conventional techniques (see Patent Documents 1 to 8 and 10 to 14) paying attention to the shape of the nozzle inner hole portion, an effect of preventing a drift of the molten steel flow can be expected to a certain degree because a turbulent flow is partially generated. There is however a problem that “deviation in discharge flow rate distribution of molten steel” occurs easily particularly in the discharge hole portion, that is, a minus flow (suction flow) occurs or when a plurality of discharge holes are provided, imbalance occurs in the flowing amount out of each discharge hole.
Description will be further made taking the immersion nozzle as an example. The nozzle has an important role of supplying molten steel into the mold uniformly. Actually, a flow of molten steel in the nozzle is provided as a drift because of flow rate control based on a slide valve. There is a possibility that this will cause a drift of molten steel in the discharge hole and will cause deterioration of cast piece quality because this has influence on the inside of the mold. Besides the flow rate control based on the slide valve, flow rate control based on a stopper and a vortex of molten steel generated in a vessel at the time of discharge of molten steel are causes of occurrence of a drift in the immersion nozzle.
The aforementioned problem can be solved to a certain degree by the shape of the nozzle inner hole portion listed in the conventional techniques. Particularly in the “immersion nozzle having a plurality of step portions” described in the aforementioned Patent Document 1, a drift suppressing effect can be obtained to a certain degree because molten steel passes through the portion where the sectional area of the nozzle is reduced by each step. The height of the step used in practice is about 5 mm. If the height of the step is made higher, the drift suppressing effect can be improved but there is a problem that the amount of passage of molten steel (throughput) is limited by decrease in sectional area of the step portion and increase in frictional resistance of the pipe wall. Also in the “nozzle having semi-spherical concave-convex portions in a surface of a molten metal flow path” described in the aforementioned Patent Document 6, the effect of preventing a drift of molten steel and the effect of suppressing deposition of alumina cannot be always satisfied.
The drift of molten steel in the nozzle inner hole portion causes a “drift of molten steel in the discharge hole portion”. The “drift of molten steel in the discharge hole portion” will be described with reference to (A) and (B) in FIG. 1. A molten steel flow a shown in (A) of FIG. 1 is not uniformly discharged from the discharge hole portion (side hole type) but drifts as represented by the solid-line arrow shown in the drawing. That is, a minus flow (suction flow) is generated. As a result, the possibility that mold powder will be involved as represented by the broken-line arrow occurs and causes deterioration of cast piece quality. Not only in the “side hole type” shown in (A) of FIG. 1 but also in a “bottom hole type” straight immersion nozzle 10b shown in (B) of FIG. 1, the molten steel flow a′ does not uniformly flow out of the discharge hole portion (bottom hole type) so that a drift is generated in the discharge hole portion as represented by the solid-line arrow shown in the drawing. Incidentally, (A) are (B) of FIG. 1 are based on the “water model experiment” of inner pipe straight immersion nozzles 10a and 10b having discharge hole portions of a “side hole type” and a “bottom hole type” respectively. This phenomenon occurs even in the case where the shape of the nozzle inner hole portion is changed to any one of shapes listed in the conventional techniques. This fact has been confirmed from the “water model experiment” performed by the present inventors.
There is also a problem that alumina is attached and deposited on a space between protrusions disposed in the molten steel flow hole portion of the immersion nozzle in accordance with the method of providing the protrusions when Al killed steel or the like is cast. If alumina is deposited so that the space between the protrusions is filled with alumina, the effect based on the provision of the protrusions is eliminated so that the drift preventing effect is spoilt. At the same time, predetermined throughput (the amount of passage of molten steel per unit time) cannot be kept because the effective sectional area of the inner hole portion is reduced. There is a disadvantage that the nozzle cannot operate. Incidentally, in the method of spraying inert gas which is one of the conventional techniques for preventing alumina from being deposited on the casting nozzle, the alumina deposition preventing effect can be expected but there is a disadvantage that melting loss in the inner surface of the nozzle discharge hole is made severe by the bubbling stirring effect of the inert gas. In addition, there is a problem that cast piece defects occur easily because pinhole defects occurs easily based on gas bubbles in accordance with the size, dispersibility, etc. of the bubbles generated. On the other hand, in the alumina-deposition-free material adapted to the nozzle, the alumina deposition preventing effect can be expected to a certain degree but it cannot be said that the required effect is accomplished.